1. Field of the Invention
The present invention relates to a color-developing sheet for pressure sensitive recording sheets using as a color-developing agent a polyvalent metal salt of aromatic carboxylic acid of which the disadvantageous properties are improved.
2. Prior Art
Pressure-sensitive recording sheets are known carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies. A colored image is formed by a color developing reaction of a electron-donating colorless dyestuff and a electron-accepting color-developing agent.
Hitherto, many kinds of electron-accepting color-developing agents are well-known. The typical color-developing agents include inorganic solid acids such as acid clay, activated clay, attapulgite (described in U.S. Pat. No. 2,712,597); substituted phenols and diphenols (described in the Japanese Patent Publication No. 9309/1985); p-substituted phenol-formaldehyde polymers (described in the Japanese Patent Publication No. 20144/1967); aromatic carboxylic acid metal salts (described in the Japanese Patent Publication Nos. 10856/1974 and 1327/1977); 2,2'-bisphenol sulfone compounds (described in the Japanese Patent Laid-Open No. 106313/1979); and so on.
Among these color-developing agents, polyvalent metal salts of aromatic carboxylic acids such as 3,6-di-(.alpha.-methylbenzyl) salicylic acid zinc salt and the like, have been widely applied for practical use since they have superior resistance of the colored image to light, humidity, heat and plasticizer.
However, the color-developing sheet containing the polyvalent metal salt of aromatic carboxylic acid has the defects that a color-developing rate is slow and a color-developing intensity is low.